1. Field of the Invention
The field of the invention is that of positioning, to be more precise that of stand-alone position determination devices (such as Global Positioning System (GPS) receivers for example) or integrated position determination devices, for example devices incorporated in mobile terminals.
In the present context, the expression “mobile terminal” means a communication terminal equipped with a position determination device, where applicable one using satellites, such as a mobile telephone, for example, or a personal digital assistant (PDA), where applicable of the type able to communicate.
2. Description of the Prior Art
Positioning methods, for example methods using satellites, comprise two steps executed sequentially in a position determination device (also referred to as a positioning device). This is known in the art.
In a first step, called the acquisition step, the device determines pseudorandom codes that modulate signals coming from satellites that are “in view” and belong to a constellation of positioning satellites and are referenced to a reference time usually called the “system time”. In the present context, the expression “constellation of positioning satellites” means a radio navigation satellite service (RNSS) positioning network, such as the GPS network, the GLONASS network or the pending GALILEO network, for example.
The signals received from the satellites that are in view are “compared” to signal replicas based on hypotheses as to the system time and to the timing frequency of the satellites, in order to deduce therefrom the pseudorandom codes modulating said received signals, or in other words to synchronize the clock of the terminal and its frequency to the clock and the frequency of each satellite in view. To this end, correlation measurements are usually effected based on pairs of temporal and frequency hypotheses.
Then, in a second step, the device determines the estimated position of the terminal in which it is installed from the pseudorandom codes acquired and navigation data contained in the signals received in particular. To be more precise, this second step consists in determining from the acquired pseudorandom codes propagation times of the signals between each of the satellites in view and the terminal, then determining, from these propagation times and navigation data contained in the signals, pseudodistances between the terminal and each of the satellites in view, and finally determining the estimated position of the terminal from these pseudodistances.
This latter determination necessitates at least one quadrilateration, and more generally a numerical solution by the least squares method with four unknowns and using at least four measurements (four measurements are necessary for solving the four unknowns). Under certain conditions only three measurements are used and one unknown is fixed, typically the altitude (Z) of the receiver, or hybridization with external measurements may be used.
The accuracy of each propagation time, and thus of each pseudodistance, determines the accuracy of the estimated position directly. Now, the accuracy of each propagation time depends on the quality of the acquisition of the pseudorandom codes from the corresponding received signal, which is dependent on the quality of said received signal.
Consequently, if at least one of the signals received from a satellite in view is of poor quality, which is a relatively frequent occurrence, especially in rough or congested environments or at the edge of a satellite coverage area, the position determined is generally subject to error. It may even happen that it is momentarily impossible to determine the position of the terminal, even though the signals coming from the other satellites in view are of good quality.
To improve on this situation, and in particular to improve the accuracy of the estimated position, it has been proposed to couple the constellations of positioning satellites to systems, known as “augmentation systems”, of the satellite-based augmentation system (SBAS) type, such as the EGNOS system, for example.
An augmentation system generally consists of ground stations and geosynchronous satellites (such as IMMARSAT and ARTEMIS, for example) for transmitting, generally by radio, augmentation data to mobile terminals provided with positioning devices.
The augmentation data is generally representative of the approximate reference time of the constellation, the approximate position of the terminal concerned and at least one radius of uncertainty associated with that approximate position. However, it may equally be representative of ephemeredes, complementary navigation data, or temporal corrections, possibly representative of disturbances induced by the ionosphere to the propagation of signals transmitted by the satellites in view.
The augmentation data is essentially used in the step of determining the estimated position and the associated radii of uncertainty, called protection level(s).
The radii of uncertainty constitute integrity data that generally represents an envelope, defined by a horizontal radius and a vertical radius, for example, centered on the associated approximate position, and in which the terminal concerned is deemed to be found.
This envelope, also called the protection radius, is used by the device to estimate its service availability relative to a requested accuracy requirement.
The larger the envelope, the less accurate the estimated position is likely to be. In other words, the radius of protection defines a “level of quality” in terms of potential position accuracy.
The accuracy of the estimated position and the accuracy of the protection radius depending on correction and integrity (or level of quality) information received, the positioning device is therefore dependent on what the augmentation system transmits to it.
Now, under certain conditions, it is essential to have an accuracy higher than that which may be obtained, at a given time, by calculating the position in accordance with the standard cited above.
Furthermore, when the terminal is located in a degraded radio environment or at the edge of a coverage area it cannot receive the augmentation data (typically correction and integrity information), which degrades positioning accuracy and prevents determination of the protection radius and thus leads to unavailability of service.
GPS code filtering techniques have been proposed for taking account of the situations cited above. However, they relate only to fixed terminals, because their convergence times are too long to enable their use in mobile terminals.
GPS phase filtering techniques have also been proposed, but necessitate not only a reference station whose position is known accurately but also permanent visibility of the satellites of the GPS constellation.
Thus an object of the invention is to improve on this situation.